I. Field of the Invention
The present invention is directed generally to improvements in the treatment of concentrated aqueous waste streams having a significant content of organic matter and, more particularly, to a process for rendering sludges of high solids content in which the solids have a relatively high biological oxygen demand (BOD).
II. Related Art
Wastewater treatment has been and continues to be a matter of great environmental importance. This includes addressing issues relating to industrial operations such as food processing plants, paper mills, and the like, in addition municipal sewage and animal waste treatment streams. Traditionally, organic matter containing excessive BOD has been treated using microbial action in a manner that separates the organic matter to form a mass of solids in the form of an activated sludge from the remainder or water fraction. The treatment problem is decidedly two-fold because the water and the sludge fractions both must be treated to be safely returned to the environment and the two fractions may contain different impurities to be removed.
Treatment typically involves digestion of the organic material through fermentation of the sludge involving aerobic or anaerobic bacterial action or some combination thereof. These processes are used to reduce or consume the chemical oxygen demand (COD) and biological oxygen demand (BOD) of the material and reduce them to an environmentally safe level in the organic materials. It is also necessary to remove undesirable inorganic materials from the water fraction which typically contains undesirable quantities of phosphorus and nitrogen compounds including phosphates and nitrates.
Problems associated with the keeping of large numbers of animals in close quarters have evolved to an acute stage in agriculture, particularly with respect to the growing requirements for treating animal waste associated with feedlots, animal barns or the like, where the organic matter to be treated is highly concentrated with respect to BOD, phosphorus and nitrogen content and the conditions for legal disposal are continually becoming more restricted. In many areas, feedlots, for example, are required to be licensed by state law or local ordinance so that the treatment and disposal of such biological wastes are carefully monitored. In Minnesota for example, feedlots having 300 or more animal units are affected and, according to 1990 Minnesota Agricultural Statistics Service data, 14,000 feedlot operators have 300 animal units (an animal unit is .about.1000 lbs of animal such as 1-1000 lb bovine or 2.5 large hogs) or more and so are in the regulated category. The cost associated with compliance with regulations affecting animal waste disposal is generally quite high and is increasing as regulations become more restrictive.
Additionally, existing processes and installations for the treatment of such wastes have generally been large scale operations having installation costs measured in terms of millions of dollars such as associated with typical municipal treatment plants, or the like. Most known treatments or processes also are generally directed to treating dilute concentrations of organic matter with relatively low BOD.
Wastewater from feedlots or animal barn grated or troughed floors, however, contains from about 0.5-10% organic solids. Even if this effluent be diluted with additional wash down water, it remains a highly concentrated biomass with respect to BOD and COD. In contrast to typical municipal wastewater treatment facilities which are designed to treat relatively dilute waste sludge having a BOD in the neighborhood of 200 mg/l or 200 parts per million (ppm). The barn or feedlot wash down effluent may have a BOD of 10,000-100,000 mg/l (ppm) or more than an order of magnitude greater than the level most existing processes are prepared to meet. Because of this, a great deal more oxygen per unit volume of wastewater is required to reduce the BOD/COD to an acceptable level and make the material more olfactorily acceptable.
Systems have been proposed for the aerobic treatment of liquid borne biowastes in which the oxygen potential has been enriched or elevated above that which normally dissolves in the liquid at atmospheric pressure. For example, Smith, in U.S. Pat. No. 4,163,712, introduces the gas into an inlet stream under pressure and turbulent conditions to produce an input stream containing oxygen in amounts greatly in excess of the equilibrium saturation value, the excess forming finely dispersed bubbles therein. This stream is caused to enter at a low level near the bottom of an unpressurized main biomass to allow the undissolved gas to bubble through the main mass when the pressure is released and to dissolve in the main body of liquid as it rises, thereby replenishing the available oxygen. In this manner, oxygenation of the main reactive biomass may be maintained at a higher level.
Such a system, however, to be practical requires an oxygen-containing gas mixture having an enriched oxygen content (i.e., greater than that of air) and preferably close to that of pure oxygen, which, of course, is too expensive to be practical in a relatively small scale system such as would be operated by an individual, for example, as it would require a continuous source of liquid oxygen or the like to produce highly enriched air. Franco (U.S. Pat. No. 4,645,603) discloses a system that uses jet ejectors to aerate an unpressurized open holding tank 5 to 20 feet in depth in which the air is supplied from an open-ended mixing tube at a pressure slightly above the pressure of the hydraulic head of the tank (at the depth of the tube) and released to bubble through the reacting biomass.
In other approaches, a part of the system is pressurized to increase the oxygen solubility. These include Stoyer (U.S. Pat. No. 4,042,494) in which oxygen is dissolved at high pressure toward the lower end of an extremely long inclined reactor pipe, preferably &gt;1000 feet long, through which the biomass travels, such that the oxygen bubbles through the agglomeration and reacts as the material is pumped along the pipe. Pollock et al, in U.S. Pat. No. 4,340,484, employ an open vertical shaft which may be upwards of 500 feet high in the form of a downcomer and riser connected in a bottom loop to attain high pressure dissolution capabilities. Air is added to the waste liquor toward the bottom of the riser portion of the vertical shaft such that, as released the dissolved oxygen equilibrium is at a maximum and thereafter oxygen is continually consumed as the material rises along the riser portion and the pressure (and oxygen equilibrium solubility) correspondingly decreases.
Additionally, systems and methods have been devised for anaerobically treating wastewater to remove phosphorus using phosphorus consuming microorganisms. U.S. Pat. No. 4,999,111, to Williamson, deals with the removal of phosphorus and nitrogen, particularly from wastewater of relatively low BOD to P ratio using both oxic and anoxic zones where nitrogen, phosphorus and other pollutants are removed. The process requires a fraction of the primary sludge to be fermented to soluble organics to provide sufficient nutrients to sustain the phosphorus consuming and denitrification systems.
It can be seen from the above that a proliferation of approaches to dealing with wastewater have been proposed, including some directed toward increasing the maximum O.sub.2 respiration rate. These processes and techniques, however, for the most part have been quite expensive to operate such as those requiring the provision of enriched air or pure oxygen or extremely large and impractical to install as those requiring extremely deep shafts or elongated tubular pipe devices. There clearly exists a need to provide a compact, low-cost system and method for treating concentrated wastewater (10,000-100,000 mg/l BOD) as exists in wash down water from feedlots and animal barns that can be applied to localized operations.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for processing concentrated wastewaters of high BOD content in a compact, low-cost system that can be designed to accommodate effluent from individual operations.
Another object of the present invention is to provide a method and apparatus for processing concentrated wastewater that can be installed at the site of the operation and connected directly thereto.
Yet another object of the present invention is to provide a method and apparatus for processing concentrated wastewater of high BOD that also is capable of biologically removing phosphorus.
A further object of the present invention is to provide a method and apparatus for processing concentrated wastewater of high BOD that also is capable of biologically removing nitrogen.
Still another object of the present invention is to provide a method and apparatus for processing concentrated wastewaters of high BOD that maintains a high level of dissolved oxygen in the primary reactor without the need to use enriched air or added oxygen.
Yet still another object of the present invention is to provide a method for processing concentrated wastewaters of high BOD content that produces an effluent suitable for spreading in fields or use as a high-protein feed.
Other objects and advantages associated with the present invention will be gleaned by those skilled in the art upon appreciation of this specification.